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Jet prop Work to underpin the British Museum's famous Round Reading Room is nearing completion, which will then allow the Great Court development to proceed. Max Soudain of Ground Engineering reports.

In the UK tourist world, the British Museum is king, second only in terms of number of visitors to Blackpool's Pleasure Beach. The museum attracted over six million visitors in 1997, hardly surprising as it houses, among other things, one of the most important collections of Egyptian artefacts outside Cairo.

And in the run up to the museum's 250th anniversary in 2003, the historic building is under going a radical transformation. The Round Reading Room, one time home of the British Library, is being fully restored and the central courtyard in which it sits will regain its intended status as the museum's focal point.

Work started on the 97M Great Court project in March, following the removal of the last of the British Library's collection to its new site next to St Pancras station. Funding includes 30M from the Millennium Commission and 15.75M from the Heritage Lottery Fund.

The main thrust of the geo-technical work for the project is the formation of jet grouted columns to underpin sections of the Reading Room and the buildings surrounding the 100m by 70m courtyard. This will allow excavation of a 6m deep basement in the southern half of the area and will also strengthen the existing foundations to take loads from the new steel and glass roof that will cover the courtyard.

In addition, columns are being placed at the locations of two lift shafts on the northern edge of the Reading Room, and to support a new south portico which will replace one demolished at the end of the last century.

First stage of the project was to demolish the now redundant British Library book stacks that hid the Reading Room for much of its working life and almost filled the courtyard to capacity. While this was going on, geotechnical contractor Keller Ground Engineering began to move on site.

As Keller associate director Bob Essler explains, this was not as simple as it might seem, as plant and equipment had to be taken apart and craned into the cramped site while demolition continued. 'Logistics are an important part of the job,' he says. Keller is using two jet grouting rigs and one mini piling rig, and has set up its batching plant at the front of the museum next to the site offices.

Unfortunately, site investigation had to be carried out before demolition, and access was restricted to just four boreholes, one in each corner of the courtyard. But Buro Happold Geotechnical Group manager Peter Scott says that they were 'pretty confident' of the geology, which is mostly fill, Thames Gravels and London Clay.

And before work could start, detailed finite element analyses had to be carried out both by Keller and by Buro Happold - structural engineer for the project - to finalise the design of the jet grouted columns. The original design called for ground anchors to give additional support for the jet grouted block, but 'modelling has shown that there appears to be no need to tie back', says Scott, although there is provision for ground anchoring if needed. Essler adds that analyses also allowed some jet grout columns to be removed from the design.

The Reading Room's copper dome is supported by a skeleton of 20 cast iron ribs, with an interior papier mache lining. The ribs sit on stone and brick foundations, in turn supported by a concrete slab between 1.8m and 3m thick.

Over 200, 1.2m diameter columns are being installed under the southern edge of the Reading Room foundations at 1m centres. Grouting is carried out at foundation level (around -4.5mOD) and columns key into the London Clay just below -10mOD. The vertical front row will be exposed during excavation, with the columns behind angled under the foundations.

These create a large block that will prevent both settlement and lateral movement of the Reading Room. The columns key into the underside of the concrete slab and are installed through the underlying fill and Thames Gravel and into the London Clay below. The columns also act as a cut off for the basement excavation.

Scott says that while jet grouting has proven its worth in treating coarse material, there was some concern at the beginning of the project over its effectiveness in clay.

Keller is using a triple fluid jet grouting system, in which a high pressure air shrouded water jet is used to cut the column and grout injected through a separate nozzle below.

Essler says that a double pass is used to deal with the stiff London Clay, and to ensure the integrity of the columns is maintained across the boundary between the gravel and the clay. As the tool is withdrawn, jetting is carried out, but without grout injection. Just above the boundary of the clay and gravel, the jets are switched off and the tool is returned to the base of the hole. All three jets are then switched on and conventional jet grouting carried out.

Essler adds that the different material properties were incorporated in the finite element analyses. This, and careful adjustment of the injection pressures during jetting, means the columns are sound.

One of the main problems has been preventing movement during jet grouting. The first few columns produced small amounts of heave in the Reading Room, and pressures were adjusted. However, Essler says that the heave was well within acceptable limits of 10mm vertical and 7mm lateral, and also shows that there is good contact and bond of the columns with the underside of the reading room foundation slab.

Keller came up with an interesting use of jet grout spoil. The company had to install slurry cut off walls 0.5m into the London Clay around the perimeter of the basement and instead of bringing in more material - again by crane - the cutoffs were built using the spoil, although Essler says it was necessary to increase the thickness of the walls from 450mm to around 600mm. Once the basement is finished, this material will be excavated and removed from site. Keller has also installed mini piles under the two lift pits that will be built behind the south portico.

Extensive instrumentation has been installed as part of the contract, to allow real time monitoring of ground movements during construction. Water cells to monitor settlement have been placed on the stone pads beneath the cast iron columns of the Reading Room on its southern side and one placed on each of the lift pits to the north. There is also one in the middle of the building to act as control and one at a remote location in the courtyard.

Tiltmeters have also been placed on the cast iron columns above the main area of jet grouting. Additionally, precise levelling and geodetic surveying is being carried out on the outside of the building in real time and all the instruments use data loggers.

And in the next few weeks, Keller will be installing three inclinometers at depth beneath the reading room and in the excavation area, piezometers and rod extensometers.

Excavation is due to finish next February and Essler says monitoring will continue throughout the operation and during construction of the basement levels.

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